Action of Light on Selenium. 323. 



been exposed to light for seven days, the tellurium was found 

 to be much more sensitive. "When exposed at a distance of about 

 half a metre, as before, to the paraffin-lamp the needle was gra- 

 dually deflected through 30 divisions of the scale, showing that 

 the resistance of the tellurium was now diminished as much with- 

 out-interposing the beaker of water as it had previously been when 

 the beaker was interposed. On making no change whatever, except 

 placing the beaker of water between the lamp and the tellurium, 

 so as to focus the light on the tellurium, the deflection of the needle 

 gradually increased to 80 and then more slowly to 100 divisions. 

 On balancing this deflection, it was found that, to bring the needle 

 to zero, it was necessary to diminish the slide-resistance by 400 

 millirus. Thus the diminution produced in the resistance by expo- 

 sure to the light of the paraffin-lamp was ^-J^-th of the whole re- 

 sistance of the tellurium. 



On exposing the selenium bar used in my experiments to the 

 direct rays of the same paraffin-lamp at the distance of 1 metre, 

 the resistance of the selenium was diminished by one fifth of its 

 whole resistance. 



From the above experiment we see that at the distance of half a 

 metre (that is, with light of four times the intensity) the change of 

 resistance in the tellurium under the same conditions is only 

 Y^OTj-th part of its whole resistance. 



On exposing the selenium to a constant source of light at 

 different distances, the change in the resistance of the selenium 

 on exposure for 10 seconds (as measured by the swing of the gal- 

 vanometer-needle) is almost exactly inversely as the distance, *. e. 

 directly as the square root of the illuminating power. This law 

 is true whether the source of light be 1 candle or an Argand lamp 

 whose illuminating power is equal to 16 candles. 



Taking the mean of a number of experiments, all of which 

 agreed pretty well together, the deflections at the several distances 

 were : — 



At £ metre. At § metre. At 1 metre. At 2 metres. 

 With Argand lamp . . ... 170 83 39 



„ candle 41 18 8 



„ candle 82 39 18 8 



Another series of experiments with a candle and Argand lamp 

 (when the illuminating power of the lamp was equal to 12 candles), 

 both at the distance of 1 metre, gave the following results : — 

 With the candle the deflection was 19 in 10 seconds. 

 „ Argand lamp „ 66 „ 



The ratio of the deflections is very nearly 1 to 3^. 



These experiments clearly show that the change in the resist- 

 ance of the selenium is directly as the square root of the illuminating 

 power. 



[Correction. — In my former paper on this subject (Phil. Mag. Feb. 1876), 

 on page 156, line 17, omit the word "opposing," and line 23, for "which 

 opposes" read " in the same direction as"; a'so on page 159, line 23, for " which 

 opposes a" read " in the same direction as the"; to the end of line 25 add "but 

 in the opposite direction."] 



Z2 



